In the reprocessing of nuclear fuels, it is a common practice to remove a multiplicity of burned-off nuclear fuel elements or slugs from the core of a nuclear reactor and to replace them with fresh nuclear fuel elements. The nuclear fuel elements can have an elongated shape and comprise a can or casing containing a fissionable material, e.g. uranium, in elemental form or in the form of a compound such as uranium oxide. The burned-out fuel elements also include various fission products which, if the casing or can is cracked or otherwise flawed, may pose a problem of contamination during subsequent transport, storage, or handling.
It has also been proposed to subject the burned-out fuel elements, upon removal of the reactor core, to an inspection to assure the integrity of the fuel element before it is placed, for example, in a transport container or a storage container for delivery to or temporary storage at a nuclear-fuel reprocessing center.
The inspection can be effected in an inspection vessel submerged in a liquid with a liquid sample being taken from the inspection vessel. The activity of at least one fission-product nuclide can then be measured and, if the fuel element is found to be intact, can be placed in a transport container for normal or routine delivery to the fuel processing center. If the fuel element is found to be damaged in some way, it can also be placed in a container and subjected to special handling, e.g. because contamination of the transport container may also pose a problem.
The process for detecting defects in fuel elements in this manner is known as the wet sipping process. In this process the inspection vessel, open at its upper and lower ends, is immersed to a depth of about 10 meters in a bath of water or boric acid and can accommodate but a single fuel element. The fuel elements are individually inserted into the inspection vessel and for each inspection stage the top of the inspection vessel is closed, an air cushion is formed and a natural or convective circulation is permitted in the bath. The fuel element becomes heated by the radioactive decay process so that any leak in the tubular casing, shell or can will release fission products into the aqueous phase.
Samples are taken from the water in the vessel and analyzed for the presence of such fission products to determine whether any such defect may exist.
Since, before sampling, each individual fuel element must be stored for a given minimum period to allow sufficient heat buildup, the process is extremely time-consuming. In addition, the contamination of the water of the bath continuously increases and poses a problem with respect to any subsequent samplings of the vessel water for fission-product nuclides.
The process is thus inexact and even efforts to overcome the lack of precision by multiple samplings and measurements for each fuel element have not completely overcome these disadvantages. As the number of samplings and measurements is increased, moreover, the time which must be expended on each fuel element likewise increases and renders the process uneconomical.
In the conventional approach, after a predetermined period of storage in the inspection vessel, each fuel element is introduced into a transport container for temporary storage and/or delivery to a fuel reprocessing center.
Experience has shown that fuel elements generally have deposits adhering to them which are capable of radioactively contaminating the environment and hence the transfer of even an intact fuel element to the transport container may serve to contaminate the latter by the adherent surface contaminants.
In the conventional system, the internal contamination of the transport container resulting from adherent deposits on the fuel elements transported therein builds up to an unsatisfactory level and the transport container must be subjected at considerable cost and effort, to an internal decontamination or cleaning process which utilizes much space in the nuclear power plant or reprocessing center, wastefully utilizes equipment and poses the threat of contamination to personnel.